Furrow or flood irrigation is probably one of the oldest known forms of irrigation and involves discharging irrigation water into and along an elongated furrow which generally slopes downwardly from an upstream end to a downstream end, the irrigation continuing until it is ensured that the irrigation water has percolated down to the root level of the plants to be irrigated. Such furrow irrigation has long been known to be inefficient seeing that the degree of percolation of the irrigation water in the initial upstream portion of the furrow is greater than in the downstream portions thereof. In order to ensure that, in these downstream portions the irrigation water percolates to the required soil depth, the flow of irrigation water along the furrow must be continued and, as a consequence, there is excessive percolation in the upstream portions. In addition, since the infiltration rate along the furrow reaches a stable value maintaining a constant flow in the furrow results in a water overflow at the end of the furrow constituting so-called "tail water loss". This is not only wasteful in terms of water consumption but the excessive percolation carries with it the additional disadvantage that, in the upstream portions the water percolating downwardly beyond the root level carry with it soluble salts thereby depleting the upper soil layers of these salts and giving rise to the pollution of the water table and a rise in its level.
It is well known that a major reason for the differential percolation of the water along the length of the furrow arises out of the fact that as the water flows down the furrow there is a steady decrease in the volume of the flow particularly in the downstream portions of the furrow. The stage during which the irrigation water flows from its upstream to its downstream end is known as the "advance stage" of furrow irrigation. Once the advance stage has been completed, i.e. the water flow will have reached the downstream end, the so-called "soaking stage" begins and continues until it is ensured that, at the downstream end the water will have percolated to the root level. By this time however, as indicated above, the water at the upstream portion will have percolated well below the root level in that region and considerable tail water loss will have occurred.
Irrigation efficiency E.sub.ff is defined as being the ratio of the amount of irrigation water (W.sub.r) percolating down to the root level along the length of the furrow to the total amount of water applied during the irrigation (W.sub.t), i.e. E.sub.ff =W.sub.r /W.sub.t.
Clearly the closer W.sub.r approaches W.sub.t the higher the efficiency of the irrigation.
In practice, it is found that irrigation efficiency E.sub.ff for furrow irrigation does not exceed 50%.
Various attempts have been made to improve the efficiency of furrow irrigation and these attempts have all been directed to decreasing the duration of the advance stage seeing that the lower this duration the lesser the degree of percolation in the upstream portions of the furrow.
Among known attempts to reduce the duration of the advanced stage can be mentioned the following:
1. a reduction in the infiltration capacity of the soil by compaction or other means; PA1 2. increasing the slope of the furrow; and PA1 3. increasing the water discharge rate into the furrow. PA1 (a) initiating an irrigation flow into an upstream end of a furrow at an initial water discharge rate; PA1 (b) continuously or periodically sensing the progress of water flow along said furrow; PA1 (c) continuously or periodically increasing the water discharge rate in response to the sensed progress of said water flow so as to ensure a substantially continuous flow in said furrow at a flow rate which lies substantially between .+-.30% of a constant value; PA1 (d) sensing the arrival of said water flow at a downstream end of said furrow; PA1 (e) reducing said water discharge rate to a reduced value in response to the sensed arrival of said water flow at a downstream end of said furrow; PA1 (f) sensing the percolation of said water flow at said downstream end down to a predetermined depth; and PA1 (g) terminating said irrigation flow in response to said sensed percolation.
Various combinations of these means have also been proposed. However, none of these hitherto proposed means have been particularly successful in increasing efficiency, thus, for example, undue increase of the furrow slope or of the water discharge rate significantly increases the danger of soil erosion. On the other hand, the use of shorter furrows so as to decrease the duration of the advance stage proves to be uneconomical.
A further proposal which has been made in an attempt to increase the efficiency of furrow irrigation has involved so called "surge irrigation ", wherein water is introduced into the upstream end in surge-like pulses of a relatively lengthy duration. During the "on" period, water flows down a section of the furrow whilst during the following "off" period, the surface of the irrigated section dries off as a result of the percolation of the water and, it is believed that the percolation capacity of that section decreases relatively speedily. During the subsequent "on" period, the irrigation water would be expected to flow along the preceding section at a relatively increased rate (owing to its supposed reduced percolation rate) and in this way it is believed that the overall duration of the advance stage is reduced. It is to be noted in this connection that with such surge irrigation there is no continuous flow of irrigation water along the furrow during the entire duration of the advance stage. In practice, however, it has not been found that surge irrigation results in any significant increase in irrigation efficiency and any theoretical explanation for the promise of success vis-a-vis actual relative lack of success is not well established. Furthermore, surge irrigation has not been found to correct, in a satisfactory manner, tail water loss.
It is an object of the present invention to provide a new and improved automated furrow irrigation control method which results in a significantly reduced advanced stage duration as well as the reduction or prevention of tail water loss and consequently, a significantly increased irrigation efficiency.